Electric double layer capacitors (EDL capacitors) exhibit a very large capacitance as compared to traditional capacitors. This characteristic is useful for many applications that require both high pulse power and relatively large energy storage, coupled together with a high cycle life. Batteries have been traditionally applied for such applications yet have fallen short in terms of pulse power delivery and cycle life. As a result of these superior qualities, EDL capacitors have mainly been developed specifically for applications that demand a high power density and cycle life. For applications where the primary concern is energy density, batteries have continued to dominate.
EDL capacitors store energy by charge separation in much the same manner as traditional capacitors. Traditional capacitors have two conductors that are separated by a dielectric material. When charged, the capacitor builds up a static charge on the surface of the conductors. In an EDL capacitor, the charge is also stored on two conductors. In this case, the conductors are made of a high surface area material such as activated carbon and the charge is stored throughout the volume of the electrodes. This provides an extremely large surface area onto which the charges can reside. The difference is that a secondary charge separation occurs within an electrolyte that fills the EDL capacitor. The ions that make up the electrolyte separate and migrate towards the microporous structure of the carbon electrode. The solvent molecules become the dielectric material and result in a charge separation on the order of nanometers. The combination of the large surface area and the microscopic charge separation are what gives the EDL capacitor its large capacitance.
There have been a number of technical developments leading to the prior art EDL capacitors commercially available today. The developments have ranged from the first devices with only a few farads capacitance to a few thousand today. The most common feature of today's EDL capacitors is the use of organic electrolytes with either activated carbon cloth wound in the shape of a cylinder or activated carbon powder processed into pellets. Organic electrolyte is used to avoid corrosion problems with the metal current collectors. In one approach, activated carbon cloth is used as a substrate onto which the metal collector can be conveniently applied. In another approach, activated carbon pellets are formed using finely ground carbon powder with a binding agent, usually Teflon. However, the limitations of these two approaches include the use of toxic and expensive electrolytes and expensive carbon electrode material for the carbon cloth type or limited capacitance for the pelletized type. Also, organic electrolytes have a peak performance when charged between 2.5 to 3 volts, as compared to 1.2 to 1.5 volts for most batteries. This makes it inconvenient to use such ultracapacitors as direct replacements for common batteries. Additional limitations of using organic electrolytes include their inherent high resistance and rendering the carbon electrode material with a low specific capacitance. The resistance of organic electrolytes can be more than 10 times greater than for aqueous electrolytes. The specific capacitance of most activated carbon materials is about half the value when using organic electrolytes as compared with using aqueous electrolytes. These limitations have prevented the widespread commercialization of capacitors in markets dominated by batteries.
The electrodes of EDL capacitors are most often made of activated carbon and utilize solid porous carbon electrodes. However, solid electrodes are brittle and prone to cracking and breaking Some currently known capacitors use pelletized carbon paste electrodes made of finely divided activated carbon to address the brittleness issue. This development allows a more simple method of manufacture. Pelletized carbon paste electrodes, however, exhibit a high rate of self-discharge, high internal resistance due to limited ion mobility within the electrodes, and limited specific capacitance due to the low concentration of electrolyte present. Some prior art capacitors using pelletized carbon paste electrodes use an electrolyte with a pH above 3.5 to reduce the rate of self-discharge. However, no optimum pH has been given. Also, in both low and high pH solutions, the rate of self-discharge for EDL capacitors is still high.
Another problem encountered with prior art EDL capacitors is the problem of corrosion posed by the use of an inorganic salt. The use of a conductive rubber material for the collector successfully addresses the problem of corrosion but at the cost of a high internal resistance. The problem of high internal resistance inherent with activated carbon material has been addressed by some capacitors in the prior art. The use of highly conductive additives mixed with carbon powder or impregnated with carbon fiber material reduces the internal resistance of EDL capacitors. For the case of using metals such as aluminum as an additive, as in some currently known capacitors, only organic electrolytes can be used. When powdered graphite or carbon black is used, as in other currently known capacitors, again, only organic electrolytes can be used. The procedure of adding highly conductive additives reduces the inter-particle resistance, yet still necessitates the use of relatively thin electrodes when carbon powder is utilized.
In view of the foregoing, there is a need for improved techniques for providing EDL capacitors that use inexpensive, non-toxic materials and address the issues of high self-discharge, corrosion and high internal resistance.
Unless otherwise indicated illustrations in the figures are not necessarily drawn to scale.